So let me ask you this. How much of a difference would there be between a OEM x-pipe which has primary cats swapped out for test pipes? And say another 'quality' x pipe? Performance wise that is...

Because honestly I want to to do that..keeping the secondary cats and resonators as to lessen sound and smell.. but at the same time gaining power.

There is a pretty substantial difference in total power and torque gain, and how the power is gained within the powerband.

The factory unit is not a true x-pipe by any means. I have cut the factory x-pipe apart to see what BMW did inside of their so called "X" and it is a joke. The internal merge is roughly 1.1" in diameter.

The secondary cats to help the smell a little bit.

Power wise as a test, I have run a full catless modified OEM unit, and the gains were very marginal at its best. Power delivery was erratic because exhaust design requires engineering and tuning to compliment the powerplant.

Deleting cats does free up some power, but that is not where all the power lies.